The present invention relates to a device for controlling the supply of secondary air in an internal combustion engine. The device of the present invention is used for an internal combustion engine of an automobile.
In the prior art, the introduction of secondary air in an internal combustion engine is controlled to improve the cleanliness of the exhaust gas. Independently prepared air is supplied, as secondary air, to the exhaust system at a position downstream of the engine outlet. This secondary air is supplied only when the opening of the throttle valve is small, such as during idling or deceleration, thus bringing a three-way catalyst to either the oxidization atmosphere state or the stoichiometrical air-fuel ratio gas acceptance state. This control is carried out by feedback-control of the carburetor in accordance with, for example, the output signal of an O.sub.2 sensor.
The above prior art control is carried out on the basis of slow air bleeding and main air bleeding in the carburetor system. In such control by air bleeding, however, fluctuations in the characteristics of the air-fuel ratio of the carburetor in the transient range from low speed running to high speed running of the internal combustion engine make it necessary to appropriately correct the air-fuel ratio to realize a constant value stoichiometrical air-fuel ratio in the gas supplied to the three-way catalyst. Failure to appropriately correct the air-fuel ratio results in a delay in control, and, accordingly, deteriorated cleanliness of the exhaust gas. For example, in the transient state from the slow air bleeding system to the main air bleeding system in the low speed and light load condition, a uniform air-fuel ratio of the gas supplied to the three-way catalyst could not be realized. Hence, the amount of the produced HC and CO would increase, and, accordingly, the exhaust gas cleanliness would deteriorate.
To counter the above problem, the pressure in the air intake pipe has been detected, and a signal representing the results of this detection has been supplied to a control circuit as a control input signal to change the control constants of the circuit. This method, however, necessitates additional devices, and, accordingly, makes the price of the entire system undesirably expensive.